1. Technical Field
The present invention relates to electronic units, electronic units to be charged, and control methods for electronic units. More particularly, the present invention relates to an electronic unit which charges a portable electronic unit, an electronic unit to be charged which is a portable electronic unit to be charged, and a control method for an electronic unit which charges a portable electronic unit.
2. Background Art
Compact, portable electronic units (electronic units to be charged), such as portable communication terminals and electronic timepieces, which are placed in a charging unit called a station and charged have recently become known. Some compact, portable electronic units which can be charged have lithium-ion secondary batteries as accumulators for accumulating electric energy. Since lithium-ion secondary batteries have advantages of a high energy density and relatively low self-discharging, they are widely used especially for compact, portable electronic units which require a high energy density such as portable telephones, camcorders, and notebook personal computers.
If a voltage equal to or higher than a so-called limit voltage is applied to a lithium-ion secondary battery, dendrite (treelike crystal) is formed, an internal short-circuit occurs, and the life of the battery is reduced. Therefore, in a general charging method, constant-current charging is applied to a lithium-ion secondary battery until the charged voltage (voltage to which the battery is charged) reaches the limit voltage, and then constant-voltage charging is applied after the charged voltage reaches the limit voltage. Details of this charging method are described, for example, in Japanese Unexamined Patent Application Publication No. Hei-5-111184.
In this charging method, however, after the charged voltage reaches the limit voltage, although the consumed current hardly changes, since the charging current is drastically reduced, wasteful power consumption occurs. Since the temperature of the battery continues to rise due to a leakage current and due to the heat of the compact, portable electronic unit caused by the operation of a charging control circuit, the compact, portable electronic unit and the lithium-ion secondary battery may deteriorate.